Control method and system of fuel cell system

ABSTRACT

A control method and system of a fuel cell system are provided. The control method includes draining the voltage of a fuel cell stack by charging a high voltage battery. In addition, the method includes draining the voltage of the fuel cell stack by connecting a fuel cell load device to the fuel cell stack, which is performed when the voltage of the fuel cell stack decreased by the first draining process is less than a predetermined first reference voltage.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority of Korean Patent ApplicationNumber 10-2014-0154114 fried on Nov. 7, 2014, the entire contents ofwhich application are incorporated herein for all purposes by thisreference.

BACKGROUND

1. Field of the Invention

The present invention relates generally to a control method and systemof a fuel cell system and, more particularly, to a control method of afuel cell system that prevents deterioration of a fuel cell stack anddiagnoses a failure in a load for draining residual voltage of the fuelcell stack.

2. Description of the Related Art

A fuel cell system applicable to a hydrogen fuel cell vehicle, a type ofeco-friendly vehicle, is composed of a fuel cell stack to generateelectric power from an electrochemical reaction of reactant gases; ahydrogen supplying system configured to supply hydrogen as fuel to afuel cell stack; an air supplying system configured to supply gasincluding oxygen as an oxidant in electrochemical reactions; and a heatand water management system configured to manage water and maintain anoptimal fuel cell stack temperature for driving by emitting heat, whichis a by-product of the electrochemical reactions therein.

FIG. 1 is an exemplary schematic view illustrating an overall fuel cellsystem. As shown in FIG. 1, the fuel cell system 100 may include a fuelcell stack 10, a fuel cell load device 20, an air blower 30, ahumidifier 40, air shutoff valves 35 and 45 in an inlet and outlet, adrain valve 42, a purge valve 44, a water trap 50, a hydrogenrecirculation device 55, a hydrogen supply valve 57, a radiator 60, anda thermostat 65. The air shutoff valves 35 and 45 in the inlet andoutlet may prevent inflow of an air to the fuel cell stack after a fuelcell vehicle shuts down. The drain valve 42 is disposed in a hydrogenexhaust line to remove generated water at an anode, and the purge valve44 adjusts hydrogen concentrations at the anode and exhausts hydrogen toan air outlet to dilute the air.

The fuel cell load device 20 that lowers the voltage of the fuel cellstack 10 to drain the voltage is connected to the fuel cell stack 10while the fuel cell vehicle is shutting down and after the fuel cellvehicle shuts down, to remove oxygen inside the fuel cell stack 10. Theoxygen flowing into the fuel cell stack 10 is removed with residualhydrogen of the anode as the fuel cell load device 20 consumes current.However, insufficient hydrogen may preclude complete consumption of theoxygen, and thus a Wakeup technique is used to periodically supplyhydrogen to the anode.

In other words, unlike an internal combustion vehicle, the fuel cellsystem 100 requires a post-processing procedure that lowers the voltageof the fuel cell stack 10 by removing residual air inside the fuel cellstack 10 after the fuel cell vehicle shuts down. The post-processingprocedure prevents deterioration of the fuel cell stack 10 to avoid thedanger of exposure to a high voltage. When voltage is formed as oxygenexists at the anode, carbon corrosion may occur at a cathode.Accordingly, it is necessary to remove the oxygen inside the fuel cellstack 10 and to prevent the additional inflow of oxygen. When additionaloxygen flows into the fuel cell stack, draining oxygen removal isnecessary. Consequently, when a vehicle shuts down, the system stops thesupply of oxygen, intentionally consumes the residual oxygen for loadcurrent using the fuel cell load device 20, and decreases the voltage.

It is an object of the present invention to provide a control method andsystem of a fuel cell system, which may decrease voltage of the fuelcell stack using a battery and a fuel cell load device, and may diagnosea breakdown of the fuel cell load device.

A control method of a fuel cell system according to an exemplaryembodiment of the present invention may include: draining the voltage ofa fuel cell stack by charging a high voltage battery; and draining thevoltage of the fuel cell stack by connecting a fuel cell load device tothe fuel cell stack when the voltage of the fuel cell stack is less thana predetermined first reference voltage (V1) as the voltage is beingdrained.

When the high voltage battery may not be charged, when a fuel cellvehicle crash is detected, or when a time duration for a voltage of afuel cell stack to reach a predetermined first reference voltage isgreater than a predetermined first reference time period, charging ofthe high voltage battery may be stopped and the second draining may beperformed. The high voltage battery may not be charged under particularcircumstances including when the high voltage battery breaks down, apower converter connected to the high voltage battery breaks down, astate of charge (SOC) of the high voltage battery is greater than apredetermined SOC, or a power source for the charging of the highvoltage battery is insufficient. Since the fuel cell load device isconnected to the fuel cell stack, when a voltage of the fuel cell stackis decreased to be less than a predetermined second reference voltage(V2), the second draining process may be terminated.

The control method may further include, adjusting a voltage of a mainbus terminal disposed between the fuel cell stack and an inverter to anacceptable minimum voltage (V3) to prevent output power of the fuel cellstack from being provided to loads excluding the fuel cell load deviceuntil a voltage of the fuel cell stack reaches a predetermined secondreference voltage (V2) according to the connection to the fuel cell loaddevice. The adjustment of the voltage of the main bus terminal to theacceptable minimum voltage (V3) may maintain the voltage of the main busterminal to be an initial value, and when the voltage of the fuel cellstack reaches the predetermined second reference voltage (V2), thevoltage of the main bus terminal may be decreased to the acceptableminimum voltage (V3).

The adjustment of the voltage of the main bus terminal to the acceptableminimum voltage (V3) may maintain the voltage of the main bus terminalto be greater than a voltage of a fuel cell stack. In addition, theadjustment of the voltage of the main bus terminal to the acceptableminimum voltage (V3) may include disconnecting the fuel cell stack fromthe main bus terminal by turning off a main relay before connecting tothe fuel cell load device. The acceptable minimum voltage (V3) may beless than the first reference voltage, and greater than a minimumvoltage for operating a power converter connected to the high voltagebattery or high voltage components connected to the main bus terminal.

Prior to the first draining process, the control method may furtherinclude stopping the supplying of air to the fuel cell stack afterincreasing the voltage of the fuel cell stack to be greater than thefirst reference voltage by supplying air to the fuel cell stack. Priorto the second draining process, the control method may further includedisconnecting the fuel cell stack from a main bus terminal disposedbetween the fuel cell stack and an inverter, by turning off a mainrelay.

A control method of a fuel cell system according to another exemplaryembodiment of the present invention may include: connecting a fuel cellload device to a fuel cell stack; and diagnosing an operation of theconnected fuel cell load device. Additionally, the diagnosing of theoperation of the fuel cell load device may include diagnosing theoperation of the fuel cell load device based on at least one amongcurrent output from the fuel cell stack, current flowing in the fuelcell load device, and a speed of decrease in voltage of the fuel cellstack.

The diagnosing of the operation of the fuel cell load device may furtherinclude diagnosing that the fuel cell load device breaks down when adifference between a set current value and a current value flowing inthe fuel cell load device or a difference between the set current valueand a value of the output current of the fuel cell stack is greater thanpredetermined tolerances, respectively, the set current value beingcalculated based on both the voltage of the fuel cell stack and aresistance value of the fuel cell load device. The tolerances may bepredetermined respectively based on a resolution of a sensor, sensingthe current value flowing in the fuel cell load device, and a resolutionof a sensor configured to sense the output current value of the fuelcell stack.

The diagnosing of the operation of the fuel cell load device based onthe speed of decrease in voltage of the fuel cell stack may includediagnosing the operation of the fuel cell load device based on both atime required for the voltage of the fuel cell stack to reach a specificvoltage after the fuel cell load device is connected to the fuel cellstack and a reference time required for the voltage of the fuel cellstack to reach the specific voltage under a normal operation of the fuelcell load device. The reference time may vary based a vehicle speed, anamount of air inflow, a water content of the fuel cell stack, acrossover state inside the fuel cell stack, or the voltage of the fuelcell stack at the moment when the fuel cell load device is connected tothe fuel cell stack.

The diagnosing of the operation of the fuel cell load device based onthe speed of decrease in voltage of the fuel cell stack may includediagnosing the operation of the fuel cell load device based on both thevoltage of the fuel cell stack when a specific time period has elapsedafter the fuel cell load device is connected to the fuel cell stack anda reference voltage that the fuel cell stack reaches after the specifictime period under a normal operation of the fuel cell load device. Thereference voltage may vary based on a vehicle speed, an amount of airinflow, a water content of the fuel cell stack, a crossover state insidethe fuel cell stack, or the voltage of the fuel cell stack when the fuelcell load device is connected to the fuel cell stack.

Additionally, the diagnosing of the operation of the fuel cell loaddevice may be performed based on at least one among a current outputfrom the fuel cell stack and a current flowing in the fuel cell loaddevice when the voltage of the fuel cell stack is greater than apredetermined voltage. The diagnosing of the operation of the fuel cellload device may also be performed based on the speed of decrease involtage of the fuel cell stack when the voltage of the fuel cell stackis less than a predetermined voltage.

The fuel cell load device may be connected to the fuel cell stack whenthe voltage of the fuel cell stack is less than a predetermined firstreference voltage as the voltage of the fuel cell stack is being drainedby charging a high voltage battery; and the diagnosing of the operationof the fuel cell load device may be terminated when the voltage of thefuel cell stack is decreased to be less than a predetermined secondreference voltage. The predetermined voltage may be greater than thepredetermined second reference voltage and less than the predeterminedfirst reference voltage.

When the voltage of the fuel cell stack is greater than thepredetermined voltage, when a difference between a set current value anda current value flowing in the fuel cell load device or a differencebetween the set current value and a value of output current of the fuelcell stack is greater than predetermined tolerances, respectively, thefuel cell load device may be determined to be broken down, the setcurrent value being calculated based on the voltage of the fuel cellstack and a resistance value of the fuel cell load device. When thevoltage of the fuel cell stack is less than the predetermined voltage,the operation of the fuel cell load device may be diagnosed based onboth a time required for the voltage of the fuel cell stack to reach aspecific voltage from starting time of the diagnosis and a referencetime required for the voltage of the fuel cell stack to reach thespecific voltage under a normal operation of the fuel cell load device.

The reference time may vary based on a vehicle speed, an amount of airinflow, a water content of the fuel cell stack, a crossover state insidethe fuel cell stack, or the voltage of the fuel cell stack when the fuelcell load device is connected to the fuel cell stack. The predeterminedvoltage may be greater than the specific voltage. When the voltage ofthe fuel cell stack is less than the predetermined voltage, theoperation of the fuel cell load device may be diagnosed based on both avoltage of the fuel cell stack when a specific time period has elapsedfrom the start time of the diagnosis and a reference target voltage whenthe specific time period has elapsed under a normal operation of thefuel cell load device.

The reference voltage may vary based on a vehicle speed, an amount ofair inflow, a water content of the fuel cell stack, a crossover stateinside the fuel cell stack, or the voltage of the fuel cell stack whenthe fuel cell load device is connected to the fuel cell stack. Thepredetermined voltage may be greater than the reference voltage. Thecontrol method may further include, when the diagnosing of the operationof the fuel cell load device is terminated, storing results based on thetermination of the diagnosis to a memory.

According to an exemplary embodiment of the present invention, thecontrol method of the fuel cell system may improve durability of thefuel cell system by draining residual oxygen of the fuel cell stack.Additionally, the method may decrease a danger of high voltage exposureby lowering the voltage of the fuel cell stack. The method may alsomaintain stability when restarting the vehicle, and may improve fuelefficiency of the vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is an exemplary schematic view illustrating an overall fuel cellsystem according to the related art;

FIG. 2 is an exemplary block diagram of a power net of a fuel cellsystem according to an exemplary embodiment of the present invention;

FIG. 3 is an exemplary block diagram of a power net of a fuel cellsystem according to another exemplary embodiment of the presentinvention;

FIGS. 4A and 4B are exemplary flow diagrams illustrating a controlmethod of a fuel cell system according to an exemplary embodiment of thepresent invention;

FIG. 5 is an exemplary block diagram of a power net of a fuel cellsystem according to an exemplary embodiment of the present invention;

FIG. 6 is an exemplary block diagram of a power net of a fuel cellsystem according to another exemplary embodiment of the presentinvention;

FIGS. 7A and 7B are exemplary views illustrating a process for drainingthe voltage of a fuel cell stack using a high voltage battery and aprocess to disable the process for draining the voltage of the fuel cellstack using the high voltage battery, respectively according to anexemplary embodiment of the present invention;

FIG. 8 is an exemplary graph illustrating variations in voltage andcurrent with time when a fuel cell is shut down according to anexemplary embodiment of the present invention;

FIG. 9 is an exemplary flow diagram illustrating a method for diagnosingan operation of a fuel cell load device using a current value detectedby a first current sensor shown in FIG. 5 according to an exemplaryembodiment of the present invention;

FIG. 10 is an exemplary flow diagram illustrating a method fordiagnosing an operation of a fuel cell load device using a current valuedetected by a second current sensor shown in FIG. 6 according to anexemplary embodiment of the present invention;

FIGS. 11A and 11B are according to an exemplary embodiment of thepresent invention graphs illustrating, when a fuel cell load device isunder a normal operation and when the fuel cell load device breaks down,a time it will take to reach a specific voltage and a voltage after aspecific time period according to variations in voltage of a fuel cellstack, respectively according to an exemplary embodiment of the presentinvention;

FIG. 12 is an exemplary flow diagram illustrating a method fordiagnosing an operation of a fuel cell load device according to anexemplary embodiment of the present invention;

FIG. 13 is an exemplary flow diagram illustrating a method fordiagnosing an operation of a fuel cell load device according to anotherexemplary embodiment of the present invention; and

FIGS. 14 and 15 are according to an exemplary embodiment of the presentinvention flow diagrams illustrating a control method of a fuel cellsystem according to other exemplary embodiments of the presentinvention.

DETAILED DESCRIPTION

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g. fuels derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example bothgasoline-powered and electric-powered vehicles.

Although exemplary embodiment is described as using a plurality of unitsto perform the exemplary process, it is understood that the exemplaryprocesses may also be performed by one or plurality of modules.Additionally, it is understood that the term controller/control unitrefers to a hardware device that includes a memory and a processor. Thememory is configured to store the modules and the processor isspecifically configured to execute said modules to perform one or moreprocesses which are described further below.

Furthermore, control logic of the present invention may be embodied asnon-transitory computer readable media on a computer readable mediumcontaining executable program instructions executed by a processor,controller/control unit or the like. Examples of the computer readablemediums include, but are not limited to, ROM, RAM, compact disc(CD)-ROMs, magnetic tapes, floppy disks, flash drives, smart cards andoptical data storage devices. The computer readable recording medium canalso be distributed in network coupled computer systems so that thecomputer readable media is stored and executed in a distributed fashion,e.g., by a telematics server or a Controller Area Network (CAN).

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. As used herein, the term “and/of”includes any and all combinations of one or more of the associatedlisted items.

Unless specifically stated or obvious from context, as used herein, theterm “about” is understood as within a range of normal tolerance in theart, for example within 2 standard deviations of the mean. “About” canbe understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%,0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear fromthe context, all numerical values provided herein are modified by theterm “about.”

Specific structural or functional descriptions in the exemplaryembodiments of the present invention disclosed in the specification orapplication are only for description of the exemplary embodiments of thepresent invention, can be embodied in various forms and should not beconstrued as limited to the embodiments described in the specificationor application.

Specific exemplary embodiments are illustrated in the drawings anddescribed in detail in the specification or application because theexemplary embodiments of the present invention may have various formsand modifications. It should be understood, however, that there is nointent to limit the embodiments of the present invention to the specificembodiments, but the intention is to cover all modifications,equivalents, and alternatives included to the scope of the presentinvention.

Although the terms first, second, etc. may be used herein to describevarious elements, these elements should not be limited by these terms.These terms are used to distinguish one element from another. Forexample, a first element could be termed a second element, and,similarly, a second element could be termed a first element, withoutdeparting from the scope of the present invention.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present. Other words used to describe therelationship between elements should be interpreted in a like fashion(e.g., “between” versus “directly between,” “adjacent” versus “directlyadjacent,” etc.).

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

Reference now should be made to the drawings, in which the samereference numerals are used throughout the different drawings todesignate the same or similar components.

FIG. 2 is an exemplary block diagram of a power net of a fuel cellsystem according to an exemplary embodiment of the present invention,and FIG. 3 is an exemplary block diagram of a power net of a fuel cellsystem according to another exemplary embodiment of the presentinvention. As shown in FIGS. 2 and 3, a fuel cell-battery hybrid systemfor a vehicle may include: a fuel cell 10 as a main power source and ahigh voltage battery (main battery) 220 as an auxiliary power source,which may be connected with each other in parallel via a main busterminal 211; a bidirectional direct current-direct current (DC/DC)converter (e.g., BHDC: Bidirectional High Voltage DC/DC Converter) 221connected to the high voltage battery 220 to adjust output power of thehigh voltage battery 220; an inverter 231 connected to the main busterminal 211 on the output side of both the fuel cell 10 and the highvoltage battery 220; a driving motor 232 connected to the inverter 231;a high voltage load 233 within the vehicle, exclusive of the inverter231 and the driving motor 232; a low voltage battery (auxiliary battery)240 and a low voltage load 241; a low voltage DC/DC converter (e.g.,LDC: Low Voltage DC/DC Converter) 242, connected between the low voltagebattery 240 and the main bus terminal 211, configured to convert a highvoltage to a low voltage; and a fuel cell load device 20.

Herein, both the fuel cell 10 as a main power source and the highvoltage battery 220 as an auxiliary power source may be connected inparallel via the main bus terminal 211 to intra-system loads such as theinverter 231, the driving motor 232, etc. The bidirectional DC/DCconverter 221 connected to the high voltage battery may be connected tothe main bus terminal 211 at the output side of the fuel cell 10, andtherefore it may be possible to adjust output power of both the fuelcell 10 and the high voltage battery 220 by adjusting a voltage of thebidirectional DC/DC converter 221 (e.g., an output voltage to the mainbus terminal).

The fuel cell 10 may include a diode 213 at an output terminal thereofto prevent back current and a relay 214 to selectively connect the fuelcell 10 to the main bus terminal 211. The relay 214 may be disposed inthe power net shown in FIG. 2 but may not be disposed in the power netshown in FIG. 3. The relay 214 in FIG. 2 may be configured to connectthe fuel cell 10 to the main bus during the idle stop/restart process ofthe fuel cell system and during the driving of the vehicle under anormal operation of the fuel cell 10 (e.g., operation without failure)and to disconnect the fuel cell 10 from the main bus upon the key-off ofthe vehicle (e.g., normal shutdown) or an emergency shutdown. The fuelcell load device 20 may be a load for draining a voltage of the fuelcell upon the start-up and shut-down of the fuel cell. The fuel cell 10and the fuel cell load device 20 may be connected to each other via afuel cell load relay 25.

The fuel cell system in FIG. 3 eliminate the relay 214, therefore it maybe possible to reduce a cost for the relay and to reduce noise causedfrom the operation of the relay 214. However, to shut off an outputpower of the fuel cell, which is a role of the relay 214 in FIG. 2, thesystem should maintain a voltage of the bidirectional DC/DC converter221 to be greater than an open circuit voltage of the fuel cell stackand limit load of both the high voltage load 233 and the low voltageload 241 within allowable output power of the high voltage battery,without the relay 214. Additionally, to reduce a time required to formhigh voltage when the fuel cell shuts down, the voltage of the fuel cell10 is required to be drained. When the relay 214 is included in thesystem, it may be possible to prevent high voltage being formed in thebus terminal through shutoff of the relay 214.

FIGS. 4A and 4B are exemplary flow diagrams illustrating a controlmethod of a fuel cell system according to an exemplary embodiment of thepresent invention. FIG. 4A illustrates a control method in which thefuel cell system may include a main relay 214 configured to prevent anoutput power of the fuel cell from being provided to a main busterminal, while FIG. 4B illustrates a control method in which the fuelcell system does not include the main relay 214. In particular, a fuelcell controller (FCU) may be configured to execute the respective stepsin the flow diagram, or multiple controllers for each component withinthe fuel cell system, which communicate with the fuel cell controllermay be configured to execute the process.

Referring to FIGS. 4A and 4B, when the fuel cell system 100 is shut down(S401), the controller may be configured to determine whether a voltageof the fuel cell stack 10 is less than a predetermined first referencevoltage (V1) (S403). When the voltage of the fuel cell stack 10 is lessthan the predetermined first reference voltage (V1), air may be suppliedto the fuel cell stack 10 (S405) and the voltage of the fuel cell stack10 may be increased until the voltage is greater than the firstreference voltage (V1) (S407), and then the supply of air to the fuelcell stack may be terminated (S409). When the voltage of the fuel cellstack is greater than the first reference voltage (V1), air may beprevented from being supplied to the fuel cell stack and the supply ofair process may be terminated (S409).

In other words, stopping the supply of air may be performed regardlessof the voltage of the fuel cell, but when the voltage of the fuel cellstack 10 is substantially low, the controller may be configured tosupply air to the fuel cell stack to increase the voltage of the fuelcell stack to be greater than the first reference voltage (V1), and thenstop the supply of air. When the fuel cell is shut down during the idlestop process, the voltage of the fuel cell stack 10 is already low, andthus it may be difficult to diagnose an operation of the fuel cell loaddevice 20. To diagnose whether the fuel cell load device is operated, itmay be necessary to operate the fuel cell load device in a state inwhich the voltage of the fuel cell stack 10 is as high as a particulardegree. Accordingly, after air is supplied to the fuel cell stack toincrease the voltage of the fuel cell stack to a particular degree, thesupply of air may be stopped.

After stopping the supply of air, the controller may be configured todetermine whether the high voltage battery 220 may be charged, orwhether the fuel cell vehicle crash has not occurred (S411). Whether thehigh voltage battery may be charged 220 may be determined based on atleast one condition selected from the group consisting of whether thehigh voltage battery 220 breaks down, whether the bidirectional DC/DCconverter 221 connected to the high voltage battery 220 breaks down,whether the SOC (State of Charge) of the high voltage battery 220 isgreater than a predetermined SOC, and whether a power source for thecharging of the high voltage battery 220 is insufficient.

In response to determining the high voltage battery 220 may be charged,the voltage of the fuel cell stack may be consumed by the charging ofthe high voltage battery 220, and thus the voltage of the fuel cellstack 10 may be decreased (S413). As the voltage of the fuel cell stack10 is decreased, the controller may be configured to determine whetherthe voltage of the fuel cell stack 10 is less than the predeterminedfirst reference voltage (V1) or whether charging time of the highvoltage battery 220 is greater than a predetermined time period (T1)(S415). When the voltage of the fuel cell stack 10 is less than thepredetermined first reference voltage (V1) or when the charging time ofthe high voltage battery 220 is greater than the predetermined timeperiod (T1), charging of the high voltage battery may be stopped (S417).

As shown in FIG. 4B, for the system that includes the main relay 214,when the voltage of the fuel cell stack 10 is less than thepredetermined first reference voltage, or when the charging time of thehigh voltage battery is greater than the predetermined time period (T1),the fuel cell stack 10 may be disconnected from the main bus terminal211 by turning off the main relay 214 (S416), and the voltage of themain bus terminal 211 may be decreased to reach a third referencevoltage (V3) that may be predetermined to be about the same as the firstreference voltage (V1) or to be less than the first reference voltage(V1) (S417). Stopping the charging of the high voltage battery 220 maybe determined based on conditions that include the voltage of the mainbus terminal 211, the maximum amount of the voltage drained from thefuel cell stack, which is attributable to the charging of the highvoltage battery, and the like. The process of draining the voltage ofthe fuel cell stack 10 attributable to the charging of the high voltagebattery 220 and the process of draining the voltage of the fuel cellstack 10 attributable to the operation of the fuel cell load device 20may not be performed simultaneously.

More specifically, to prevent the output the power of the fuel cellstack 10 to loads (e.g., high voltage battery, auxiliary machinery,etc.) exclusive of the fuel cell load device 20, the voltage of the mainbus terminal 211 may be decreased to the predetermined third referencevoltage. Further, the fuel cell stack 10 may be connected to the fuelcell load device 20 (S419). The operation of the fuel cell load device20 may be diagnosed while the fuel cell load device 20 is beingconnected to the fuel cell stack 10 (S421). In other words, thecontroller may be configured to diagnose or determine whether the fuelcell load device 20 is being operated according to the purpose of thedesign thereof without failure.

When the voltage of the fuel cell stack is decreased to be less than apredetermined second reference voltage (V2) (S423), the diagnosis may beterminated and results of the diagnosis may be stored in a memory(S425). Then, the voltage of the main bus terminal 211 may be decreasedto the predetermined third reference voltage (V3) by adjusting thevoltage of the power converter, the bidirectional DC/DC converter 221(S427). The voltage of the main bus terminal 211 may be decreased to thepredetermined third reference voltage to facilitate determining whetherto turn on a warning light, etc. according to the result of thediagnosis of the fuel cell load device 20, which may indicate whetherthe fuel cell load device breaks down, read from the memory at the nextstart-up of the vehicle after the shut-down.

The first reference voltage (V1) may be predetermined to be within thevoltage operation range of the power converter 221. Additionally, thefirst reference voltage may be predetermined to be a substantially lessvalue for the fuel efficiency of the fuel cell vehicle, while it may bepredetermined to be a substantially greater value for the diagnosis ofwhether the fuel cell load device 20 breaks down. The third referencevoltage (V3) may be a reference voltage for decreasing the voltage ofthe main bus terminal 211 to maintain stability of restarting, and maybe predetermined to be a minimum voltage to drive the loads (e.g.,auxiliary machinery, etc.) inside the fuel cell system 100 within thevoltage operation range of the power converter 221. The first referencevoltage (V1) may be about the same as the third reference voltage (V3)or greater than the third reference voltage (V3). Additionally, thethird reference voltage (V3) may be greater than the minimum voltage tooperate the power converter 221 connected to the high voltage battery220 or high voltage components connected to the main bus terminal 211.

The fuel cell system 100 that does not include the main relay 214 mayomit the process of decreasing the voltage of the main bus terminal 211(S417) before stopping the charging of the high voltage battery 220, orif the system performs the step of S417, the system may be configured toprevent the power of the fuel cell stack 10 to be output to the loadsincluding the high voltage battery 220, exclusive of the fuel cell loaddevice 20.

FIG. 5 is an exemplary block diagram of a power net of a fuel cellsystem according to an exemplary embodiment of the present invention,and FIG. 6 is an exemplary block diagram of a power net of a fuel cellsystem according to another exemplary embodiment of the presentinvention. The description for the same components and configurationsillustrated in FIG. 3 will be omitted. Referring to FIG. 5, the fuelcell system in FIG. 5 may further include a first current sensor 27configured to sense a size of current flowing to the fuel cell loaddevice 20. Current sensors adapted to the current range flowing to thefuel cell load device may be used as the first current sensor 27. Thefuel cell system illustrated in FIG. 6 may include a second currentsensor 29 configured to sense output current of the fuel cell stack 10,instead of the first current sensor 27 to sense the current flowing tothe fuel cell load device 20. In particular, since the output current ofthe fuel cell stack 10 may have a greater range than the current flowingto the fuel cell load device 20, the second current sensor may becapable of high-resolution current sensing.

FIGS. 7A and 7B are exemplary views illustrating a process for drainingthe voltage of a fuel cell stack 10 using a high voltage battery 220 anda process to disable the process for draining the voltage of the fuelcell stack 10 using the high voltage battery 220, respectively. Drainingof the voltage of the fuel cell stack 10 using the high voltage battery220 may be performed until the voltage reaches the predetermined thirdreference voltage (V3). The voltage of the main bus terminal 211 may bedecreased over time, but the acceptable minimum voltage may be limitedto the third reference voltage (V3) (e.g., the voltage of the main busterminal may be adjusted to an acceptable minimum voltage (V3)). Thethird reference voltage or the acceptable minimum voltage (V3) may bepredetermined to be the lower value between the minimum operatingvoltage of the bidirectional DC/DC converter 221 and the minimum drivingvoltage of the high voltage load 233.

FIG. 7B shows when the voltage of the main bus terminal 211 maintainedover time, or in which the minimum voltage limit of the main busterminal 211 varies and the voltage of the main bus terminal isdecreased based on the varying minimum voltage limit. The minimumvoltage limit may be determined to the greater value between the voltageof the fuel cell stack 10 and the third reference voltage (V3) tomaintain the voltage of the main bus terminal 211 greater than theoutput voltage of the fuel cell stack 10, whereby the output power ofthe fuel cell stack 10 may be prevented from being provided to the loadsexcluding the fuel cell load device 20.

FIG. 8 is an exemplary graph illustrating variations in voltage andcurrent with time when a fuel cell is shut down. Section 1 is a sectionin which the voltage of the fuel cell stack 10 is drained by thecharging of the high voltage battery 220, and section 2 is a section inwhich the voltage of the fuel cell stack 10 is drained using the fuelcell load device 20. In the section 2, the operation of the fuel cellload device 20 may be diagnosed. Further, in the section 2, the mainrelay 214 may be turned off to prevent charging of the high voltagebattery 220 by the fuel cell stack 10 or provide the output power of thefuel cell stack 10 from being provided to the auxiliary machinery.Additionally, decreasing the voltage of the main bus terminal 211 usingthe bidirectional DC/DC converter 221 may be disabled, and the voltagecontrol 2 illustrated in FIG. 7B is performed.

FIG. 9 is an exemplar flow diagram illustrating a method for diagnosingthe operation of the fuel cell load device 20 using a current valuedetected by the first current sensor 27 shown in FIG. 5. FIG. 10 is anexemplary flow diagram illustrating a method for diagnosing theoperation of the fuel cell load device 20 using a current value detectedby the second current sensor 29 shown in FIG. 6.

Referring to FIGS. 9 and 10, the control method of the fuel cell systemaccording to an exemplary embodiment of the present invention mayinclude diagnosing an operation of the fuel cell load device 20connected to the fuel cell stack 10, and diagnosing the operation of thefuel cell load device 20 may include calculating an estimated currentvalue (I_(est)) based on both the voltage of the fuel cell stack 10 anda resistance value of the fuel cell load device 20 (S701, S801). Whenthe difference between the estimated current value (I_(est)) and a valueof the current flowing in the fuel cell load device 20 (I_(real-load))or the difference between the estimated current value (I_(est)) and avalue of the output current of the fuel cell stack 10(I_(real-fuelcell)) is greater than a predetermined tolerance (K, M)(S703, S803), the controller may be configured to determine that thefuel cell load device 20 breaks down (S705, S805). The predeterminedtolerance (K, M) may be set differently based on the resolution of thefirst current sensor 27 of the fuel cell load device 20 and theresolution of the second current sensor 29 of the fuel cell stack 10.

FIGS. 11A and 11B are exemplary graphs illustrating, when the fuel cellload device is under a normal operation (e.g., when no failure or breakdown occurs) and when the fuel cell load device breaks down, a timerequired to reach a specific voltage and a voltage after a specific timeperiod based on variations in voltage of the fuel cell stack,respectively.

Referring to FIG. 11A, for diagnosing the operation of the fuel cellload device 20 based on the speed of the decrease in voltage of the fuelcell stack 10, the operation of the fuel cell load device 20 may bediagnosed based on both the time required for the voltage of the fuelcell stack 10 to reach a specific voltage (V11) after the fuel cell loaddevice 20 is connected to the fuel cell stack 10 and a reference time(T_f) required for the voltage of the fuel cell stack 10 to reach thespecific voltage (V11) under the normal operation of the fuel cell loaddevice 20.

Referring to FIG. 11B, for diagnosing of the operation of fuel cell loaddevice based on the speed of the decrease in voltage of the fuel cellstack, the operation of the fuel cell load device 20 may be diagnosedbased on both the voltage of the fuel cell stack 10 when a specific timeperiod (T11) has elapsed after the fuel cell load device 20 is connectedto the fuel cell stack and a reference voltage (V_f) that the fuel cellstack 10 reaches when the specific time period (T11) has elapsed underthe normal operation of the fuel cell load device 20.

In other words, after the fuel cell load device 20 is operated, theoperation of the fuel cell load device may be diagnosed based on whetherthe time required for the voltage of the fuel cell stack to reach thespecific voltage (V11) is greater than the reference time period andwhether the voltage of the fuel cell stack after the specific timeperiod (T11) is greater than the reference voltage. The time required toreach the specific voltage (V11) and the voltage that the fuel cellstack may reach after the specific time period (T11) may be variablebased on the initial voltage of the fuel cell stack when the fuel cellload device 20 is operated (that is, the voltage of the fuel cell stackwhen starting to provide the output power of the fuel cell stack 10 tothe fuel cell load device 20). Additionally, the failure criterion ofthe fuel cell load device 20 may be variable based on the initialvoltage of the fuel cell stack when the fuel cell load device isoperated. Both the time required to reach the specific voltage and thevoltage that the fuel cell stack may reach may be increased as the speedof the vehicle increases. The times may also be increased when the airflow is detected and when the fuel cell stack 10 is in a dry-out statein which humidification of the fuel cell stack 10 is insufficient (e.g.,internal resistance of the fuel cell increases). Further, the times maybe decreased when hydrogen crossover occurs. Consequently, inconsideration of the external environment and the degree ofdeterioration of the fuel cell stack (10), the reference voltage and thereference time period may be changed. In particular, when the speed ofthe vehicle, the amount of the air flow, the degree of the dry-out, andthe hydrogen crossover rate are increased, the diagnosis of theoperation of the fuel cell load device 20 may be disabled.

Referring to FIGS. 11A and 11B, it may be possible to diagnose whetherthe fuel cell load device is under the normal operation or it breaksdown, according to the time required to reach the specific voltage oraccording to the voltage after the specific time period, which may bechanged by the initial voltage of the fuel cell stack 10 (e.g., thevoltage when the diagnosis is started). The criterion of determining thenormal condition or failure condition may have offsets as L1, L2.

FIG. 12 is an exemplary flow diagram illustrating a method fordiagnosing the operation of the fuel cell load device according to anexemplary embodiment of the present invention, and FIG. 13 is anexemplary flow diagram illustrating a method for diagnosing theoperation of the fuel cell load device according to another exemplaryembodiment of the present invention.

FIGS. 12 and 13 shows the method for diagnosing the failure in the fuelcell load device 20 in which the current sensors 27, 29 are not used incontrast with the method illustrated in FIGS. 9 and 10, which diagnosesthe failure in the fuel cell load device 20 using the value detected bythe current sensor. When the system does not include the current sensors27, 29, the failure in the fuel cell load device 20 may be diagnosedusing the speed of the decrease in voltage of the fuel cell stack 10.

In other words, after the fuel cell load device 20 is connected to thefuel cell stack 10, the controller may be configured to determinewhether the time required for the voltage of the fuel cell stack toreach the specific voltage (V11) is greater than the reference timeperiod, or whether the voltage of the fuel cell stack 10 is greater thanthe reference voltage when the specific time period has elapsed, andthus the controller may be configured to determine whether the fuel cellload device 20 breaks down.

Referring to FIG. 12, the diagnosing of the operation of the fuel cellload device 20 may include: determining whether the elapsed time afterthe connection to the fuel cell load device 20 is greater than aspecific time limit (S901); determining whether the actual voltage ofthe fuel cell stack is greater than the reference voltage (V_f) when theelapsed time is greater than the specific time limit (S903); anddiagnosing the operation of the fuel cell load device 20 (S905, S907).When the elapsed time is greater than the specific time limit and whenthe actual voltage of the fuel cell stack 10 is greater than thereference voltage (V_f) of the normal condition, the fuel cell loaddevice 20 may be determined to be broken down, that is, not in thenormal operating condition (S905), while when the actual voltage of thefuel cell stack 10 is less than the reference voltage (V_f), the fuelcell load device 20 may be determined to be operating normally (S907).

Furthermore, referring to FIG. 12, the diagnosing of the operation ofthe fuel cell load device 20 may include: after the connection to thefuel cell load device 20, determining whether the voltage of the fuelcell stack 10 reaches a specific voltage (V11, V12) (S901); determiningwhether the time required for the voltage of the fuel cell stack 10 toreach the specific voltage (V11, V12) is greater than the reference timeperiod (T_f) (S903); and diagnosing the operation of the fuel cell loaddevice 20 (S905, S907). When the voltage of the fuel cell stack 10reaches the specific voltage (V11) and when the time required for thevoltage of the fuel cell stack 10 to reach the specific voltage (V11) isgreater than the reference time period (T_f) of the normal condition,the fuel cell load device 20 may be determined to be broken down, thatis, not in the normal operating condition (S905), while when the timerequired for the voltage of the fuel cell stack 10 to reach the specificvoltage (V11) is less than the reference time period (T_f), the fuelcell load device 20 may be determined to be operating normally (S907).

Referring to FIG. 13, the diagnosing of the operation of the fuel cellload device 20 may include: setting a reference voltage (V_f) of thefuel cell stack 10, which is the reachable voltage when a specific timeperiod (T11) has elapsed after the connection to the fuel cell loaddevice 20 under the normal operation of the fuel cell load device 20(S1001); determining whether the specific time period has elapsed afterthe connection to the fuel cell load device 20 (S1003); and diagnosingthe operation of the fuel cell load device (S1007, S1009) according tothe difference between the voltage of the fuel cell stack 10 and thereference voltage (V_f) (S1005) when the specific time period (T11) haselapsed after the connection to the fuel cell load device.

In other words, first, the fuel cell load device 20 may be connected tothe fuel cell stack 10, and the reference voltage may be previously set(S1001), which is, when the specific time period (T11) has elapsed afterthe connection, the voltage that the fuel cell stack may reach as thevoltage is decreased by the normal operation of the fuel cell loaddevice 20. Then, the controller may be configured to determine whetherthe specific time period (T11) has elapsed (S1003), and the voltage ofthe fuel cell stack 10, detected after the specific time period (T11),may be compared with the reference voltage (S1005). When the differencebetween the detected voltage and the reference voltage is greater thanthe offset (L2), the fuel cell load device may be determined to bebroken down (S1007).

Additionally, referring to FIG. 13, the diagnosing of the operation ofthe fuel cell load device 20 may include: setting a reference timeperiod (T_f), which is the time required for the voltage of the fuelcell stack 10 to reach a specific voltage (V11) under the normaloperation of the fuel cell load device 20 (S1001); determining whetherthe voltage of the fuel cell load device 20 reaches the specific voltage(V11) (S1003); and diagnosing the operation of the fuel cell load device(S1007, S1009) according to the difference between the reference timeperiod and the actual time required for the voltage of the fuel cellstack 10 to reach the specific voltage (V11) (S1005).

In other words, first, the fuel cell load device 20 may be connected tothe fuel cell stack 10, and the reference time period (T_f) may bepreviously set (S1001), which is the time it will take to decrease thevoltage of the fuel cell stack 10 to the specific target voltage afterthe connection under the normal operation of the fuel cell load device20. Further, the controller may be configured to determine whether thevoltage of the fuel cell stack is decreased to the specific voltage(S1003), and the time required to decrease the voltage of the fuel cellstack 10 to the specific voltage may be compared with the reference timeperiod (S1005). When the difference between the reference time periodand the time required to decrease the voltage of the fuel cell stack 10to the specific voltage is greater than the predetermined tolerance(L1), the fuel cell load device may be determined to be broken down(S1007). When the difference between the reference time period and thetime required to decrease the voltage of the fuel cell stack 10 to thespecific voltage is less than the predetermined tolerance (L1), the fuelcell load device 20 may be diagnosed to be operating normally (S1009).

The reference target voltage after the specific time period and thereference time period required to reach the specific voltage may varyaccording to the voltage of the fuel cell stack 10 when the fuel cellload device 20 is connected to the fuel cell stack 10. In addition, thereference time period and the reference voltage may increase as thespeed of the fuel cell vehicle increases. Additionally, when the airflow is detected and when the fuel cell stack 10 is in a dry-out statein which humidification of the fuel cell is insufficient, the referencetime period and the reference voltage may increase. Further, as hydrogencrossover rate of the membrane of the fuel cell that comprises the fuelcell stack 10 is increased, the reference time period and the referencevoltage may decrease.

FIGS. 14 and 15 are exemplary flow diagrams illustrating a controlmethod of a fuel cell system according to other exemplary embodiments ofthe present invention. The steps from S1103 to S1107 in FIG. 14 and thesteps from S1203 to S1207 in FIG. 15 are the same as the steps from S801to S805 in FIG. 10. The steps from S1109 to S1113 in FIG. 14 and thesteps from S1209 to S1215 in FIG. 15 are the same as the steps from S901to S907 in FIG. 12 and the steps from S1003 to S1009 in FIG. 13,respectively. Accordingly, descriptions about the formerly mentionedsteps will be omitted.

The control method illustrated in FIGS. 14 and 15 changes the method fordiagnosing the operation of the fuel cell load device 20 accordingly asthe voltage of the fuel cell stack is greater than or less than apredetermined voltage (V4) when starting the diagnosis of the operationof the fuel cell load device 20. In other words, the voltage of the fuelcell stack 10 may be measured and compared with the predeterminedvoltage (V4) (S1101, S1201). When the voltage of the fuel cell stack 10is greater than the predetermined voltage (V4), the estimated currentvalue (I_(est)) may be calculated based on both the voltage of the fuelcell stack 10 and the resistance value of the fuel cell load device 20(S1103, S1203). Then, when the difference between the estimated currentvalue (I_(est)) and the output current value of the fuel cell stack 10(I_(real-fuelcell)) is greater than a predetermined tolerance (K)(S1105, S1205), the fuel cell load device 20 may be diagnosed to afailure (S1107, S1207).

Furthermore, when the voltage of the fuel cell stack 10 is less than thepredetermined voltage (V4), the operation of the fuel cell load device20 may be diagnosed using the method illustrated in FIG. 12 or 13. Inother words, when the voltage of the fuel cell stack 10 is less than thepredetermined voltage (V4), the operation of the fuel cell load device20 may be diagnosed based on both the time required for the voltage ofthe fuel cell stack to reach the specific voltage after the fuel cellload device 20 is connected to the fuel cell stack 10 (e.g., elapsedtime) and the reference time period (T_f), which is the time requiredfor the voltage of the fuel cell stack to reach the specific voltageunder the normal operation of the fuel cell load device 20.

Additionally, when the voltage of the fuel cell stack 10 is less thanthe predetermined voltage (V4), the operation of the fuel cell loaddevice 20 may be diagnosed based on both the voltage of the fuel cellstack when the specific time period has elapsed after the fuel cell loaddevice 20 is connected to the fuel cell stack 10 and the referencevoltage (V_f) that the fuel cell stack 10 reaches when the specific timeperiod has elapsed under the normal operation of the fuel cell loaddevice 20.

Diagnosing the operation of the fuel cell load device 20 using thecurrent sensor may be more accurate than the diagnosing the operation ofthe fuel cell load device 20 based on the degree of the decrease involtage with time. However, considering the range that the currentsensor may be configured to measure, when the size of current flowing inthe fuel cell load device 20 is substantial, the operation of the fuelcell load device 20 may be diagnosed using the current sensor, whilewhen the current flowing in the fuel cell load device 20 is minimalenough to be undetectable by the current sensor, the operation of thefuel cell load device 20 may be diagnosed by the degree of decrease involtage with time.

The predetermined voltage (V4) may be set to be greater than both thespecific voltage (V11) and the reference voltage (V_f) that the fuelcell stack 10 reaches when the specific time period (T11) has elapsedunder the normal operation of the fuel cell load device 20.Additionally, as shown in FIG. 8, the predetermined voltage (V4) mayhave the value between the first reference voltage (V1) and the secondreference voltage (V2). Additionally, the predetermined voltage (V4) maybe greater than L2, the tolerance (offset). As described above, sincethe method measuring the current may be more accurate than the methodmeasuring the speed of the voltage decrease, the predetermined voltage(V4) may be the reference voltage that makes the method measuring thespeed of the voltage decrease useful.

According to an exemplary embodiment of the present invention, thecontrol method of the fuel cell system may improve durability of thefuel cell system by draining residual oxygen of the fuel cell stack.Further, the method has an effect of eliminating a danger of highvoltage exposure by lowering the voltage of the fuel cell stack.Additionally, the method may maintain stability when restarting thevehicle, and may improve fuel efficiency of the vehicle.

Although the exemplary embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

What is claimed is:
 1. A control method of a fuel cell system,comprising: draining, by a controller, the voltage of a fuel cell stackby charging a high voltage battery; and draining, by the controller, thevoltage of the fuel cell stack by connecting a fuel cell load device tothe fuel cell stack when the voltage of the fuel cell stack is less thana predetermined first reference voltage (V1) as the voltage is beingdrained.
 2. The control method of claim 1, wherein when the high voltagebattery may not be charged, when a fuel cell vehicle crash is detected,or when a time required for a voltage of a fuel cell stack to reach apredetermined first reference voltage is greater than a predeterminedfirst reference time period, charging of the high voltage battery isstopped and the second draining process is performed,
 3. The controlmethod of claim 2, wherein the high voltage battery may not be chargedunder particular circumstances that include when the high voltagebattery breaks down, when a power converter connected to the highvoltage battery breaks down, when a state of charge (SOC) of the highvoltage battery is greater than a predetermined SOC, and when a powersource for the charging of the high voltage battery is insufficient. 4.The control method of claim 1, wherein as the fuel cell load device isconnected to the fuel cell stack, when a voltage of the fuel cell stackis decreased to be less than a predetermined second reference voltage(V2), the second draining process is terminated.
 5. The control methodof claim 1, further comprising: adjusting, by the controller a voltageof a main bus terminal disposed between the fuel cell stack and aninverter to an acceptable minimum voltage (V3) to prevent output powerof the fuel cell stack from being provided to loads excluding the fuelcell load device until a voltage of the fuel cell stack reaches apredetermined second reference voltage (V2) based on the connection tothe fuel cell load device.
 6. The control method of claim 5, wherein theadjusting of the voltage of the main bus terminal to the acceptableminimum voltage (V3) maintains the voltage of the main bus terminal tobe an initial value, and when the voltage of the fuel cell stack reachesthe predetermined second reference voltage (V2), decreases the voltageof the main bus terminal to the acceptable minimum voltage (V3).
 7. Thecontrol method of claim 5, wherein the adjusting of the voltage of themain bus terminal to the acceptable minimum voltage (V3) maintains thevoltage of the main bus terminal to be greater than a voltage of a fuelcell stack.
 8. The control method of claim 5, wherein the adjusting ofthe voltage of the main bus terminal to the acceptable minimum voltage(V3) includes: disconnecting, by the controller, the fuel cell stackfrom the main bus terminal by turning off a main relay before connectingto the fuel cell load device.
 9. The control method of claim 5, whereinthe acceptable minimum voltage (V3) is less than the first referencevoltage, and is greater than a minimum voltage for operating a powerconverter connected to the high voltage battery or high voltagecomponents connected to the main bus terminal.
 10. The control method ofclaim 1, further comprising: stopping, by the controller, the supply ofair to the fuel cell stack after increasing the voltage of the fuel cellstack to be greater than the first reference voltage by supplying air tothe fuel cell stack.
 11. The control method of claim 1, furthercomprising: disconnecting, by the controller, the fuel cell stack from amain bus terminal disposed between the fuel cell stack and an inverter,by turning off a main relay.
 12. The control method of claim 5, whereinthe acceptable minimum voltage (V3) is less than the first referencevoltage, and is greater than a minimum voltage for operating a powerconverter connected to the high voltage battery or high voltagecomponents connected to the main bus terminal.
 13. The control method ofclaim 6, wherein the acceptable minimum voltage (V3) is less than thefirst reference voltage, and is greater than a minimum voltage foroperating a power converter connected to the high voltage battery orhigh voltage components connected to the main bus terminal.
 14. Thecontrol method of claim 7, wherein the acceptable minimum voltage (V3)is less than the first reference voltage, and is greater than a minimumvoltage for operating a power converter connected to the high voltagebattery or high voltage components connected to the main bus terminal.15. A control method of a fuel cell system, comprising: connecting, by acontroller, a fuel cell load device to a fuel cell stack; anddiagnosing, by the controller, an operation of the connected fuel cellload device; wherein the diagnosing of the operation of the fuel cellload device includes: diagnosing, by the controller, the operation ofthe fuel cell load device based on at least one selected from the groupconsisting of: current output from the fuel cell stack, current flowingwithin the fuel cell load device, and a speed of decrease in voltage ofthe fuel cell stack.
 16. The control method of claim 15, wherein thediagnosing of the operation of the fuel cell load device includes:diagnosing, by the controller, a break down of the fuel cell load devicewhen a difference between a set current value and a current valueflowing in the fuel cell load device or a difference between the setcurrent value and a value of the output current of the fuel cell stackis greater than predetermined tolerances, respectively, the set currentvalue being calculated based on both the voltage of the fuel cell stackand a resistance value of the fuel cell load device.
 17. The controlmethod of claim 16, wherein the tolerances are predeterminedrespectively based on a resolution of a sensor configured to sense thecurrent value flowing in the fuel cell load device, and a resolution ofa sensor configured to sense the output current value of the fuel cellstack.
 18. The control method of claim 15, wherein the diagnosing of theoperation of the fuel cell load device based on the speed of decrease involtage of the fuel cell stack, includes: diagnosing, by the controller,the operation of the fuel cell load device based on both a time requiredfor the voltage of the fuel cell stack to reach a specific voltage afterthe fuel cell load device is connected to the fuel cell stack and areference time required for the voltage of the fuel cell stack to reachthe specific voltage under a normal operation of the fuel cell loaddevice.
 19. The control method of claim 18, wherein the reference timevaries based on a vehicle speed, an amount of air inflow, a watercontent of the fuel cell stack, a crossover state inside the fuel cellstack, and the voltage of the fuel cell stack when the fuel cell loaddevice is connected to the fuel cell stack.
 20. The control method ofclaim 15, wherein the diagnosing of the operation of the fuel cell loaddevice based on the speed of decrease in voltage of the fuel cell stack,includes: diagnosing, by the controller, the operation of the fuel cellload device based on both the voltage of the fuel cell stack when aspecific time period has elapsed after the fuel cell load device isconnected to the fuel cell stack and a reference voltage that the fuelcell stack reaches after the specific time period under a normaloperation of the fuel cell load device.
 21. The control method of claim20, wherein the reference voltage varies based on a vehicle speed, anamount of air inflow, a water content of the fuel cell stack, acrossover state inside the fuel cell stack, and the voltage of the fuelcell stack when the fuel cell load device is connected to the fuel cellstack.
 22. The control method of claim 15, wherein the diagnosing of theoperation of the fuel cell load device is performed based on at leastone selected from the group consisting of: a current output from thefuel cell stack and a current flowing in the fuel cell load device whenthe voltage of the fuel cell stack is greater than a predeterminedvoltage.
 23. The control method of claim 15, wherein the diagnosing ofthe operation of the fuel cell load device is performed based on thespeed of decrease in voltage of the fuel cell stack when the voltage ofthe fuel cell stack is less than a predetermined voltage.
 24. Thecontrol method of claim 15, wherein: the connecting of the fuel cellload device to the fuel cell stack is performed when the voltage of thefuel cell stack is less than a predetermined first reference voltage asthe voltage of the fuel cell stack is being drained by the charging ahigh voltage battery; and the diagnosing of the operation of the fuelcell load device is terminated when the voltage of the fuel cell stackis decreased to be less than a predetermined second reference voltage,wherein the predetermined voltage is greater than the predeterminedsecond reference voltage and less than the predetermined first referencevoltage.
 25. The control method of claim 22, further comprising:diagnosing, by the controller, that the fuel cell load device breaksdown, the set current value being calculated depending on the voltage ofthe fuel cell stack and a resistance value of the fuel cell load devicewhen the voltage of the fuel cell stack is greater than thepredetermined voltage, when a difference between a set current value anda current value flowing in the fuel cell load device or a differencebetween the set current value and a value of output current of the fuelcell stack is greater than predetermined tolerances, respectively. 26.The control method of claim 23, wherein when the voltage of the fuelcell stack is less than the predetermined voltage, the operation of thefuel cell load device is diagnosed based on both a time required for thevoltage of the fuel cell stack to reach a specific voltage from startingtime of the diagnosis and a reference time required for the voltage ofthe fuel cell stack to reach the specific voltage under a normaloperation of the fuel cell load device.
 27. The control method of claim26, wherein the reference time varies based on a vehicle speed, anamount of air inflow, a water content of the fuel cell stack, acrossover state inside the fuel cell stack, and the voltage of the fuelcell stack when the fuel cell load device is connected to the fuel cellstack, wherein the predetermined voltage is greater than the specificvoltage.
 28. The control method of claim 23, further comprising:diagnosing, by the controller, the operation of the fuel cell loaddevice based on both a voltage of the fuel cell stack when a specifictime period has elapsed from the start time of the diagnosis and areference target voltage when the specific time period has elapsed undera normal operation of the fuel cell load device when the voltage of thefuel cell stack is less than the predetermined voltage.
 29. The controlmethod of claim 28, wherein the reference voltage varies based on avehicle speed, an amount of air inflow, a water content of the fuel cellstack, a crossover state inside the fuel cell stack, or the voltage ofthe fuel cell stack when the fuel cell load device is connected to thefuel cell stack, wherein the predetermined voltage is greater than thereference voltage.
 30. The control method of claim 15, furthercomprising: when the diagnosing of the operation of the fuel cell loaddevice is terminated, storing, by the controller, results according tothe termination of the diagnosis to a memory.
 31. The control method ofclaim 23, wherein: the connecting of the fuel cell load device to thefuel cell stack is performed when the voltage of the fuel cell stack isless than a predetermined first reference voltage as the voltage of thefuel cell stack is being drained by the charging a high voltage battery;and the diagnosing of the operation of the fuel cell load device isterminated when the voltage of the fuel cell stack is decreased to beless than a predetermined second reference voltage.